1. Field of the Invention
The present invention relates to wireless communication networks and more particularly relates to a method for performing a cell selection in case of a handover from a TDD cell to a FDD cell or vice-versa during coexistence of LTE TDD/FDD networks.
2. Description of the Related Art
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. With the increase in the number of wireless technology subscribers, there is a dearth in the available network resources that can be allocated to a subscriber. This has led to the introduction of a many new techniques as well and methods for allocating more users or providing more resources to each subscriber, including the use of increased bandwidth or additional bands. At the same time, mobile communication systems such as 3rd Generation systems (3G) and 4th Generation (4G) also known as (Long Term Evolution (LTE) systems provide enhanced technologies. These 3G and 4G systems enable higher spectral efficiencies and allow for higher data rates and cell capacities. The demands are growing in both directions of transmission, i.e. for the Downlink (DL), i.e. for transmission from the network infrastructure to a mobile transceiver, as well as in the Uplink (UL), i.e. in the direction from the mobile transceiver to the network infrastructure.
In most of the conventional mobile access networks two operation modes are used such as Frequency Division Duplex (FDD) and Time Division Duplex (TDD). FDD uses a paired spectrum with one spectral region or bandwidth for the UL transmission and another spectral region or bandwidth for the DL transmission. TDD uses one piece of spectrum, one spectral region or bandwidth for UL transmission and DL transmission. The availability of spectrum is one of the most important criteria for choosing the mode to be used.
Interworking between FDD/TDD cells in LTE networks will be of increasing importance for operators that have spectrum for both LTE modes. It will allow operators to seamlessly offer mobile broadband services on FDD spectrum and TDD spectrum, increasing capacity and improving consumer experience. Many operators with FDD-LTE networks also own spectrum suitable for TDD-LTE services.
As multimode devices supporting both FDD and TDDLTE become more common, network operators can take full advantage of their spectrum allocations and deploy TDD-LTE as an integral part of their overall mobile broadband service. Apart from these, many operators around the globe are preparing for joint deployment of TDD and FDD based systems where TDD based systems can provide the freedom to configure variable number of resources for downlink and uplink transmissions. However, having smooth interworking between the two modes poses a challenge.
In the existing method, where TDD and FDD cells are jointly deployed, when a User Equipment (UE) moves across the TDD cells and FDD cells (in connected mode) within a given area served by a Base Station (eNodeB or eNB), handover is performed by the UE. When the UE is camped on the FDD cell and network (eNB) sends a handover command to the UE for performing handover to the TDD cell with Evolved Absolute Radio Frequency Channel Number (EARFCN) and target cell ID (TDD cell ID). Then UE starts a T304 timer and lower layers start synchronizing to the DL transmission of the target cell (TDD cell). Further, the UE tries for the target cell (TDD cell) till the expiration of the T304 timer. Upon the expiration of the T304 timer, the UE starts a Radio Resource Control (RRC) connection re-establishment procedure and further starts a T311 timer for performing cell selection.
Further, the UE performs a band scan of either TDD cell or FDD cell based on implementation and on expiry of the timer T311. The UE moves to an idle mode and starts performing cell selection on last camped cell (can be either FDD or TDD cell).
In the existing system, the UE performs blind scan of either TDD or FDD band, which may not be efficient in real time scenarios, where the UE may be in FDD/TDD only area. In certain scenarios, it may also happen that when the UE sends a measurement report to the eNB. During the time the UE receives the handover message, and then the UE may move to some other area where the corresponding mode (either FDD or TDD mode) cells are unavailable. In certain situations, may be due to network error or sub-optimal cell deployments, the network may send a wrong cell id (non-existent or very weak) to the UE for performing the handover from TDD cell to the FDD cell or vice-versa. In case, if the network sends the wrong cell id, the UE performs the scan on the cell id (received from the network), which may lead to a wastage in battery power consumption of the UE.